The present invention broadly relates to the field of strapping machines and, in particular, is concerned with a new and improved construction of a band clamping device for a machine for wrapping or strapping a band, strip, ribbon or the like, especially a plastic band, --hereinafter conveniently referred to as a band-- around sundry piece goods, such as typically boxes, crates, containers, cartons packages and so forth.
The strapping machine of the present invention broadly is of the type comprising a fixed or stationary clamping jaw and a movable clamping jaw guided for movement towards and away from the stationary clamping jaw. The movable clamping jaw is articulated at an end hinge of a first element of a toggle lever, a second element of which can be rocked by means of a drive about a further end or terminal hinge. Both clamp or clamping jaws possess essentially flat clamping surfaces engaging at the band and hold such band against the action of traction or tension forces.
Particularly, when working with plastic bands the band clamping device of strapping machines is of special significance. On the one hand, the clamping device should securely hold the tensioned band, i.e., must be capable of withstanding the tension force after completion of the actual band tensioning operation without allowing for any appreciable band slip, and, on the other hand, the band should not be damaged by the clamping operation.
According to a state-of-the-art clamping device of the previously mentioned type, the movable clamp or clamping jaw is subdivided into two sections which are articulated with a respective first element of a toggle lever. The sections of the movable clamp jaw of the prior art device are displaceable along a guide extending at right-angles with respect to the clamping surface of the fixed or stationary clamp jaw. With this known device practically the entire clamping force must be exerted and also maintained by both toggle levers. The traction- or clamping force, which can be resisted by purely a clamping action, is generally dependent upon the product of the coefficient of friction between the band and the clamping jaw surface and the clamping force. On the other hand, with the presently used high tear-resistant plastic bands there is strived for a particularly great tension force, but there is only available a relatively low coefficient of friction. Moreover, these plastic bands are comparatively sensitive to external damage inasmuch as their tear resistance rapidly diminishes to unacceptable values when their longitudinal fibrous structure is exposed to external forces. An improvement of the relationship between the tension force and the clamping force by providing the clamping surfaces of the clamp jaws with barb-like protuberances which claw into the band (i.e., a transition from a force locking action to a form locking action between the band and the clamp jaws) is only possible to a limited extent due to the sensitivity of the band as concerns its becoming damaged.
Hence, if the strived for tension force is to be actually realized, then the prior art devices had to have the toggle lever designed and adjusted in such a manner that such toggle lever was capable of producing the requisite clamping force. However, in practice this result can be hardly realized. Either the toggle lever had to be adjusted such that in the clamping position of the movable clamp jaw it was almost completely extended, but due to manufacturing tolerances and also because of thickness tolerances of the band such however was practically not possible. Or else, however, the toggle lever and its drive had to be designed so robustly that even in the case of dimensional deviations of the components of the device and the band there could be produced the required clamping force before attaining the completely extended position of the toggle lever.